Pre-implantation genetic screening (PGS)

PGS is a technique for testing whether embryos have any problems with their chromosomes, which is thought to be the most common reason IVF treatment fails. This page will introduce you to what the treatment involves, the risks and how effective it is.

What is PGS?

Embryos with abnormal chromosomes often end in miscarriage, a failed treatment cycle or the child may have a condition like Down’s Syndrome.

During PGS, a single cell or a small number of cells is removed from the embryo. The DNA of these cells is then tested to see whether they have any chromosomal abnormalities. Only embryos without chromosomal abnormalities are placed back in the womb.

Who might be recommended to have PGS?

Older women are more likely to have eggs with the wrong number of chromosomes, so traditionally it’s been offered to women over 37. Abnormal chromosomes are thought to be the main reason why older women have difficulties conceiving and are more likely to have a miscarriage or a baby with Down’s Syndrome.

Your doctor may also recommend PGS if you have a family history of chromosome problems, if your sperm is at risk of carrying abnormal chromosomes, or if you’ve had several miscarriages or failed IVF attempts without explanation.

However, there is limited evidence to show that PGS benefits these groups (see more below.)

What causes abnormal chromosomes?

Abnormal chromosomes happen very frequently during the normal production of eggs, sperm and during embryo development. When the cells of eggs, sperm and embryos divide this can lead to too many or too few chromosomes, or with missing or added pieces of DNA.

Does PGS work?

In the past, some clinics recommended PGS to older women with a history of miscarriage or failed IVF cycles, men whose sperm was at risk of having chromosomal abnormalities or people with a family history of chromosome problems. However, to date there is little evidence showing it improves success rates for these groups.

There have been some small clinical trials which have shown that PGS can improve IVF success rates for women under 37 with no history of miscarriage or failed IVF cycles.

Until larger trials have been run and we have more evidence, there’s no guarantee that PGS can improve your chances of pregnancy.

How safe is it for the embryos?

Because PGS involves removing a cell or number of cells from an embryo this can cause damage to the embryo and prevent it from developing once it has been transferred into the womb.

Also research has shown that often the cells in an embryo are not chromosomally identical (called mosaicism). PGS relies on test results from one or a small number of cells being representative of the embryo as a whole. As this is not always the case, it may be possible for an embryo to give an abnormal test result when it is in fact capable of producing a healthy pregnancy.

Are there any other risks?

However, PGS also has some treatment specific risks. This includes the possibility of a misdiagnosis, although modern PGS techniques are very accurate. There’s also the risk that if all the embryos are found to have abnormal chromosomes there won’t be any embryos to put back in the womb. This is especially likely for older women.

Can I have PGS on the NHS?

No, PGS is not currently funded by the NHS. It can also be expensive so you should talk to your clinic about likely costs and make sure your clinician clearly explains why PGS might be appropriate for you.

What’s my chance of having a baby with PGS?

Unfortunately it’s not possible for us to provide accurate success rates for PGS treatment. Because many of the people who have PGS are older, have a history of miscarriage or other fertility problems, success rates vary considerably depending on the individual.

There are also far fewer cycles of PGS than IVF – in 2013 for example there were 300 cycles of PGS to around 60,000 cycles of IVF - which means success rates are likely to change considerably from year to year.